DC Series Motor MCQ Quiz - Objective Question with Answer for DC Series Motor - Download Free PDF

Explanation:

In a DC Series Motor, at Low or Light Load, Why is the Torque Low?

Definition: A DC series motor is a type of direct current (DC) motor where the field winding is connected in series with the armature winding. This configuration ensures that the same current flows through both the armature and the field winding. The torque produced in a DC series motor is proportional to the product of the armature current and the magnetic flux generated by the field winding. The relationship between the load, current, flux, and torque is a critical factor in understanding the behavior of the motor under different load conditions.

Correct Option Analysis:

The correct option is:

Option 1: Due to the low armature current and low field flux.

This is the correct explanation for the low torque in a DC series motor at light or low load. The torque in a DC series motor is given by the formula:

T ∝ Φ × I_a

Where:

• T is the torque.
• Φ is the magnetic flux produced by the field winding.
• I_a is the armature current.

At light or low load, the motor requires less torque to overcome the load. As a result, the armature current (I_a) is low. Since the field winding in a series motor is directly connected in series with the armature, the field flux (Φ) is also reduced because it depends on the armature current. The reduced flux and low armature current together result in a significantly lower torque output. Thus, the torque decreases due to the combined effect of low armature current and low field flux.

Detailed Explanation:

1. **Torque Equation in a DC Series Motor**:

The torque in a DC motor is generated by the interaction of the magnetic field and the armature current. The relationship can be expressed as:

T = k × Φ × I_a

Here, k is a constant that depends on the design of the motor. In a series motor, since the field winding is connected in series with the armature, the flux (Φ) is directly proportional to the armature current (I_a) up to the point of core saturation.

2. **Effect of Light Load**:

When the load on the motor is light, the required torque is minimal. As a result, the motor draws less current from the supply. The reduced current leads to a decrease in the field flux (Φ), since the field winding is in series with the armature. With both the armature current (I_a) and the field flux (Φ) being low, the torque produced by the motor is also low.

3. **Performance at Low Load**:

At low load, the motor operates at higher speeds because the back EMF (E_b) approaches the supply voltage (V). The reduced current (I_a) in the armature and field winding results in lower magnetic flux (Φ), which, combined with the low armature current, causes the torque to be minimal. This behavior is a characteristic of DC series motors and highlights their dependency on load conditions for efficient operation.

Conclusion:

The low torque at light load in a DC series motor is primarily due to the low armature current and the corresponding low field flux. This explanation aligns with the correct option (Option 1).

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 2: Due to the saturation of the field cores.

This option is incorrect. While core saturation can affect the performance of a DC motor, it is not the primary reason for low torque at light load. Saturation occurs when the magnetic field in the core reaches its maximum level and cannot increase further, even with an increase in current. At light load, the current (and hence the flux) is low, so saturation does not occur.

Option 3: Due to the high field flux.

This option is also incorrect. At light load, the armature current is low, resulting in low field flux, not high field flux. High field flux would require a higher armature current, which is not the case under light load conditions.

Option 4: Due to the high armature current.

This option is incorrect because, at light load, the armature current is low. High armature current occurs only under heavy load conditions, where the motor needs to produce more torque to overcome the load.

Conclusion:

Understanding the behavior of DC series motors under different load conditions is essential for their proper application and operation. At light load, the low armature current and corresponding low field flux result in low torque, as explained in the correct option. The other options misinterpret the underlying principles and do not accurately describe the motor's behavior in this scenario.

Torque in a DC Series Motor at Saturation

Definition: In a DC series motor, torque is initially proportional to the square of the armature current under unsaturated conditions. However, as the motor approaches magnetic saturation, the relationship between torque and armature current changes due to the limitation in further increase of flux.

Working Principle: The torque (T) produced in a DC series motor is generally given by the expression:

T ∝ Φ × I_a

Here:

• T = Torque
• Φ = Magnetic flux produced by the field winding
• I_a = Armature current

Under normal unsaturated conditions, the magnetic flux (Φ) is directly proportional to the armature current (I_a). As a result, the torque becomes proportional to the square of the armature current:

T ∝ Φ × I_a ∝ I_a²

However, as the motor operates at higher currents, the magnetic circuit of the motor approaches saturation. Once saturation occurs, further increases in armature current do not lead to a proportional increase in flux (Φ). Instead, the flux becomes nearly constant, and the torque becomes proportional to the armature current only:

T ∝ I_a

Correct Option Analysis:

The correct option is:

Option 1: Armature current only

At the point of saturation, the magnetic flux (Φ) produced by the field winding becomes constant due to the limitations in further magnetizing the core material. Therefore, the torque is no longer dependent on the flux and becomes directly proportional to the armature current (I_a). This is why, under saturation, the torque in a DC series motor is proportional to the armature current only.

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 2: Both armature current and field flux

Under normal (unsaturated) conditions, torque in a DC series motor is proportional to both the armature current and the magnetic flux (Φ × I_a). However, once saturation occurs, the flux becomes constant and no longer varies with armature current. Hence, this option is incorrect under saturation conditions.

Option 3: Field flux only

This option is incorrect because the torque in a DC series motor is never solely dependent on the field flux (Φ). Torque is always a product of flux and armature current (Φ × I_a). Therefore, even under saturation, armature current is essential in determining the torque.

Option 4: The square of the armature current

This option is correct only under unsaturated conditions, where flux is proportional to armature current (Φ ∝ I_a) and torque becomes proportional to the square of the armature current (T ∝ I_a²). However, under saturation conditions, flux becomes constant, and torque is no longer proportional to the square of the armature current.

Conclusion:

At the point of saturation in a DC series motor, the torque becomes proportional to the armature current (I_a) only. This is due to the magnetic saturation of the core material, which prevents further increases in flux despite increases in armature current. Understanding this behavior is crucial for analyzing the performance characteristics of DC series motors in various operating conditions.

Explanation:

Problem Statement: A 220 V, 7 HP series motor is mechanically coupled to a fan and draws 25 A and runs at 300 rpm when connected to a 220 V supply with no external resistance. The torque required by the fan is proportional to the square of the speed. The motor resistances are given as Ra = 0.6 Ω and Rse = 0.4 Ω. We need to determine the power delivered to the fan. (Assume 1 HP = 746 W).

Solution:

Step 1: Understanding the given data:

• Rated Voltage, V = 220 V
• Current, I = 25 A
• Speed, N = 300 rpm
• Torque required by the fan is proportional to the square of the speed (T ∝ N²).
• Armature resistance, Ra = 0.6 Ω
• Series field resistance, Rse = 0.4 Ω
• Total resistance (Rtotal) = Ra + Rse = 0.6 Ω + 0.4 Ω = 1 Ω
• 1 HP = 746 W

Step 2: Calculate the back EMF (Eb):

The back EMF (Eb) in a series motor can be calculated as:

Eb = V - I × Rtotal

Substitute the given values:

Eb = 220 - (25 × 1)

Eb = 220 - 25 = 195 V

Step 3: Calculate the power delivered to the fan:

The mechanical power developed by the motor is given by:

Pmech = Eb × I

Substitute the values of Eb and I:

Pmech = 195 × 25 = 4875 W

Convert this power to HP:

Pmech (in HP) = Pmech (in W) ÷ 746

Pmech (in HP) = 4875 ÷ 746 ≈ 6.54 HP

Step 4: Verify the proportionality of torque and speed:

Since the torque required by the fan is proportional to the square of the speed (T ∝ N²), the mechanical power delivered to the fan must align with this torque-speed relationship. The calculated power of 6.54 HP is consistent with the given operating conditions of the motor and fan system.

Conclusion:

The power delivered to the fan is 6.54 HP.

Correct Option: Option 1

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 2: 36 HP

This value is significantly higher than the calculated power of 6.54 HP. Such a high power output is not possible given the motor’s operating conditions and the current drawn by it. This option is incorrect.

Option 3: 4880 HP

This is an unrealistic value for the power delivered by a 7 HP motor. The motor is mechanically coupled to a fan, and the calculated power output (6.54 HP) is within the expected range. This option is incorrect.

Option 4: 0.15 HP

This value is far too low compared to the calculated power output of 6.54 HP. It does not match the motor’s operating conditions or the current drawn by it. This option is incorrect.

Conclusion:

Based on the analysis, the correct answer is Option 1: 6.54 HP. The other options are either unrealistic or do not match the given data and conditions.

The most suitable method to measure the resistance of the series field winding of a DC series motor is the Kelvin double bridge method.

Kelvin Double Bridge

• The series field winding of a DC motor has low resistance, typically in the milliohm range.
• The Kelvin double bridge is specifically designed to measure low resistances accurately.

Measurement of resistance, inductance, and capacitance

Explanation:

To determine the efficiency of a traction motor, various testing methods can be employed. The correct method, in this case, is the Field's test. Let’s delve into why this method is appropriate and why the other options are less suitable for this purpose.

Correct Option Analysis:

The correct option is:

Option 1: Field's test

Field's Test: The Field's test is a recognized method for determining the efficiency of traction motors, particularly for series motors used in electric traction. This test is also known as the regenerative method of testing.

Working Principle: The Field's test involves using two identical motors, where one acts as a generator and the other as a motor. The motor under test (MUT) is mechanically coupled to the generator. The generator is driven by the MUT, and the electrical output of the generator is fed back to the motor. This setup allows for the energy to circulate within the system, with minimal external power input. The losses in the system are measured to determine the efficiency of the traction motor.

Advantages:

• Allows for the accurate measurement of the efficiency of the motor under actual load conditions.
• Energy-efficient testing method as it recirculates energy between the motor and generator.
• Reduces the need for large external power sources, making it cost-effective.

Disadvantages:

• Requires two identical motors, which may not always be available.
• Complex setup and instrumentation are needed for accurate measurements.

Applications: Field's test is widely used for testing traction motors in electric locomotives, electric vehicles, and other applications where series motors are used for traction purposes.

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 2: Acceleration test

The acceleration test is used to determine the performance characteristics of traction motors, such as their acceleration capabilities, torque, and speed. While this test provides valuable information about the motor's dynamic performance, it is not primarily used to determine efficiency.

Option 3: Swinburne's test

Swinburne's test is a method used to determine the efficiency of shunt and compound motors. It is an indirect method where the motor is run at no-load conditions, and the losses are measured. This test is not suitable for series motors used in traction applications as it does not account for the varying load conditions experienced by traction motors.

Option 4: Retardation test

The retardation test, also known as the retardation method or running down test, is used to determine the moment of inertia and frictional losses of rotating machines. It is not primarily used to determine the efficiency of traction motors.

Conclusion:

Understanding the appropriate testing methods for different types of motors is crucial for accurate efficiency determination. The Field's test is the most suitable method for determining the efficiency of traction motors, particularly series motors, due to its ability to simulate actual load conditions and its energy-efficient testing approach. The other methods mentioned are valuable for specific purposes but do not provide the same level of accuracy and relevance for traction motor efficiency testing.

Explanation:-

• A traction motor is an electric motor used for propulsion of a vehicle, such as locomotives (trains).
• DC series motor is used as traction motor.

Field test for series motor:

• This test is applicable to two similar series motor.
• One of the machines is run as a generator while the other as a motor.
• Therefore input power is only losses in the machines.
• Gives efficiency of DC series machine.

Hopkinson’s or Regenerative or Back To Back Test:

• Two identical DC shunt machines are coupled mechanically and tested.
• One of the machines is run as a generator while the other as a motor.
• Therefore input power is only losses in the machines.
• Performed at rated speed.
• Temperature rise and commutation qualities can be observed.

Retardation or running down test:

• Applicable to shunt motors and generators.
• Used for finding the stray losses.
• Machine is speeded up slightly beyond its rated speed and then supply is cut off from the armature while keeping the field excited.
• Armature will slow down and its kinetic energy is needed to meet rotational losses. i.e., friction and windage losses.

Swinburne’s Test:

• This test is performed no load.
• The rated speed is adjusted by the shunt field resistance.
• As it is a no load test, it cannot be done on a dc series motor.
• Give efficiency of DC machine.

The correct answer is option 2):(1500 rpm)

Concept:

The back EMF of the DCSeries motor

E_b = \(P×ϕ × N× Z\over 60\times A\)

A = 2

where

E_b is the back emf  in volt

ϕ is flux per pole in weber

Z is the number of the conductors

A is the parallel path

N is the speed in RPM

Calculation:

Given 

P = 4

Z = 200

ϕ = 20 × 10^-3 Wb

I = 20 A

E_b = 200 V

Eb = \(P×ϕ × N× Z\over A\)

 200= \( 4\times 20 \times 10 ^{-3}\times 200 \times N \over 2\times 60\)

N = \(400 \times 60 \over 800 \times 20 \times 10 ^{-3}\)

= 1500 rpm

Concept of  Characteristics of DC Series Motor:

For a DC series motor, the torque is inversely proportional to the speed. The torque sped curve is, therefore, a rectangular parabola. The speed-torque relation in a DC shunt and the series motor is as shown:

• DC series motor has high starting torque; This is one of the most required characteristics of DC series motor.
• Speed control of DC series motor is easy and various methods are available for this.
• The size of the DC series motor is small. (i.e. small size and high-power rating).
• This motor can handle overload easily.
• Regenerative braking can be applied to DC series motor, But with some modifications.
• The maintenance cost required is less.

Concept:

In a DC series machine, both field and armature winding is connected in series. 

Therefore, field current will be equal to armature current.

Torque produced ∝ I_f × I_a

i.e., T ∝ ϕ . I_a

∵ ϕ ∝ I_a (in DC series motor)

∴ T ∝ I²_a

or, √T ∝ I_a

we also know that,

\(N \propto \frac{1}{\phi} \propto \frac{1}{I_a} \propto \frac{1}{\sqrt T}\)

∴ \(N\propto \frac{1}{\sqrt T}\)

Therefore, curve will be rectangular hyperpolar.

Therefore, correct option will be (2)

In series motor flux is directly proportional to line current flowing to the motor.

And also torque is directly proportional to flux and current.

So torque will be directly proportional to the square of the current.

Calculation:

τ ∝ ϕ I ∝ I²

where, ϕ = flux 

I = current

Then \( \frac{{{\tau _2}}}{{{\tau _1}}}\; = \frac{{I_2^2}}{{I_1^2}}\)

\( \frac{{{\tau _2}}}{{{\tau _1}}} = \;\frac{{{{30}^2}}}{{{{40}^2}}} = \;\frac{{900}}{{1600}} = \;\frac{9}{{16}}\)

\( {\tau _2} = \;\frac{9}{{16\;}}\;{\tau _1}\)

% change in torque = \( \frac{{{\tau _2} - {\tau _1}}}{{{\tau _1}}} \times 100\;\% \) 

 \( \Delta \tau \) =\( \;\;\frac{{\frac{9}{{16}}{\tau _1} - {\tau _1}}}{{{\tau _1}}} \times 100\% \;\)

\(\Delta \tau \) = \(\;\; - \frac{7}{{16}} \times 100\% \)

\(\Delta \tau \) = \( - 43.75\% \)

(Negative sign indicates that the torque is reduced from the original value.)

Answer is 43.75%

Concept: Torque current relation in DC machines:

T = K_a ϕ Ia

i.e., it is proportional to the product of flux and armature current.

In DC series matrix, field current and armature current are equal. Therefore,

T ∝ Ia² 

\(\frac{\delta(T)}{\delta T} = 2 \frac{\delta (Ia)}{\delta Ia}\)

∴ \(\frac{\delta T}{\delta Ia} = 2\)

or \(\frac{\delta Ia}{\delta T} = \frac{1}{2}\)

In DC shunt motor, field current is constant.

∴ T ∝ Ia

∴ \(\frac{\delta T}{\delta Ia}=1\)

In compound motor, it will initially behave as series motor and when saturation sets in, it will behave like shut motor. Hence, current to torque ratio will lie b/w 0.5 to 1.

Hence, the corresponding ratio will be minimum in DC series motor.

Therefore, correct option is (1).

Concept:

Electrical motor efficiency is defined that the ratio of mechanical power developed to the total electrical power input.

In the case of DC motor, total electrical power input = VI_L

Where V is the supply voltage and I_L is load current.

Mechanical power developed = E_bI_a

Where E_b is back emf and I_a is armature current.

So, efficiency is given by \( = \frac{{{E_b}{I_a}}}{{V{I_L}}}\)

Application:

In dc series motor, armature current = load current.

Efficiency \( = \frac{{{E_b}}}{V}\)

Therefore, the ratio of back emf (E_b) to supply voltage (V) indicates efficiency.

Concept:

In a DC series motor, the series field winding is connected in series with armature winding i.e., it is carrying high armature current.

Therefore, series field winding will have low resistance value.

The connection of DC series motor is given below.

In DC series motor, I_a = I_f

∵ we know that, torque τ 

τ = K_aϕI_a

Here ϕ ∝ I_a

∴ τ ∝ I_a²

Also, E_b = V_t - I_a(R_a + R_se)

\(\therefore I_a=\frac{V_t-E_b}{(R_a+R_{se})}\)

If the supply voltage is reduced then the armature current will reduce.

As Ia ∝ ϕ, if the armature current decreases then flux decreases.

E_b ∝ Nϕ or N ∝ Eb / ϕ

If the flux will decrease then the speed will increase.

Therefore, the correct option is (2).

When AC supply is given to DC series motor:

• An AC supply will exert unidirectional torque because the direction of armature current and field current reverses at the same time and the direction of rotation will as it is.
• Due to the presence of alternating current, eddy currents are induced in the yoke and field cores which causes excessive heating of the yoke and field cores.
• Power factor becomes low because of high inductance formed by the field and armature circuit.
• There is sparking at the brushes of the DC series motor.
  Important Points

Universal Motor or AC Series Motor:

• A universal motor is categorized under a commutator type motor.
• In order to overcome the drawbacks of operation with AC supply, some modifications are made in a DC series motor so that it can work even on the AC current. They are as follows:

• The field core is made up of the material having a low hysteresis loss.
• Field core is laminated to reduce the eddy current loss.
• The area of the field poles is increased to reduce the flux density which results, the iron loss and the reactive voltage drop are decreased.
• Increased the number of conductors in the armature circuit for getting the required torque.
• For reducing the effect of the armature reaction and improving the commutation process compensating winding used.
• Compensating winding used in field or stator slot as shown.

​

• The axis of the compensating winding is 90 degrees with the main field axis.
• If the compensating winding is connected in series with both the armature and the field, then, it is called Conductively compensated.

​

• If the compensating winding is short-circuited itself, the motor is said to be inductively compensated.

​

• It is used usually for rating up to 1 hp.
• The characteristic of the Universal motor is similar to that of the DC series motor.
• By interchanging connections of the fields with respect to the armature, the direction of rotation can be altered.
• Speed control of the universal motors is obtained by solid-state devices such as TRIAC.
• This motor is most suitable for applications requiring high speed up to 20,000 rpm.
• Application: Domestic appliances, Portable drill machine, table fan, hair drier, etc

The correct answer is option 2):(Its speed vs. armature current characteristic is a straight line.)

Concept:

DC Series motor is a variable flux machine:

When the load on the shaft of the motor is increased, the armature current also increases. Hence, the flux in a series motor increases with the increase in the armature current and vice-versa

DC Series motor never runs unloaded:

The DC series motor attains dangerously high speed when we run it on no load. The main reason of overspeeding is that at no load the flux produced by the field winding is very less and the reduced flux cause overspeeding of the motor. The speed of the motor is inversely proportional to the flux

DC series motor speed is proportional to the load current:

DC motors are relatively simple machines: when the load on the motor is constant, speed is proportional to supply voltage. And when supply voltage is constant, speed is inversely proportional to the load on the motor.

Speed vs. armature current characteristic of DC series motor:

In this motor, the current passing through the field winding is the same as that in the armature.

For a DC series motor, the torque is inversely proportional to the speed. Its speed vs. armature current characteristic is a parabola and not a straight line. So option 2 is wrong

• DC series motor has high starting torque;
• This is one of the most required characteristics of DC series motor.
• Speed control of DC series motor is easy and various methods are available for this.
• The size of the DC series motor is small. (i.e. small size and high-power rating).
• This motor can handle overload easily.
• Regenerative braking can be applied to DC series motor,
• But with some modifications.
• The maintenance cost required is less.